A solid-solid phase change material (S-SPCM) can store and release a specific amount of latent heat during its phase transition. In this regard, poly(styrene-co-maleic anhydride) (SMA)-graft-fatty acids (FA) copolymers were synthesized as novel S-SPCMs for thermal energy storage (TES). The chemical structures of the SMA-g-FA copolymers were characterized by proton nuclear magnetic resonance (H-1 NMR) and Fourier transform infrared (FT-IR) spectroscopy techniques. The phase transformations of the copolymers form crystalline phase to amorphous phase were monitored using polarized optical microscopy (POM). The latent heat TES (LHTES) properties, thermal cycling reliability, and thermal stability of the S-SPCMs were investigated by differential scanning calorimetry and thermogravimetric analysis methods. The SMA-g-FA copolymers produced as S-SPCMs showed solid-solid phase transitions at about 40 degrees C-60 degrees C range and had latent heat storage and release ability between 84 and 127 J/g, respectively. The S-SPCMs had stable chemical structures and reliable LHTES characteristics even after 5,000 thermal cycling. They had reasonable thermal conductivity value changed in the range of 0.15-0.19 W/mK. Furthermore, it was concluded that the SMA-g-FA copolymers can be considered as promising S-SPCMs for TES utilizations. POLYM. ENG. SCI., 59:E337-E347, 2019. (c) 2019 Society of Plastics Engineers